Reception systems of radio communication systems are divided broadly into two systems, that is, the super-heterodyne system which processes a received high frequency signal after converting it into an intermediate-frequency signal by performing the frequency conversion, and the direct conversion system which processes a received high frequency signal after converting it directly into a base band signal by performing the frequency conversion. In these reception systems, a receiver for the direct conversion system (hereinafter, referred to as a direct conversion receiver) has a smaller number of external components because an intermediate frequency (IF) stage is unnecessary in comparison with that of a receiver of the super-heterodyne system, so that the direct conversion receiver is low in cost. Moreover, because the direct conversion receiver has a relatively simpler circuit configuration, the direct conversion receiver is suitable for a multiband receiver, a multimode receiver and the like. Owing to these reasons, recently the direct conversion receiver has been used for many radio communication systems.
In the direct conversion receiver, when the frequency of a received high frequency signal is denoted by fRF and the frequency of a local signal to be supplied to a mixer (mixing machine) for performing the frequency conversion of the high frequency signal to the base band signal is denoted by fLo, the frequency fRF is equal to the frequency fLO. Consequently, the output signal of the mixer also includes a direct current (DC) component. As a result, the DC component is also input into a gain-controlled amplifier (GCA) provided at the later stage of the mixer for the amplitude adjustment of the base band signal.
In a radio communication device, especially in a mobile telephone, the signal level of a received signal is very weak, for example, about a minus hundred-odd dBm, and, accordingly, it is necessary to amplify the weak signal level to a signal level of about a minus ten-odd dBm by a gain-controlled amplifier. Consequently, the gain-controlled amplifier cannot deal with the problem in a one-stage configuration, and the gain-controlled amplifier is generally configured to be a multi-stage connection that has the maximum gain thereof about 60 dB. When a DC offset at the input stage of the gain-controlled amplifier of the multi-stage connection and DC offsets to be generated at each stage are transferred to the subsequent stages as they are, the DC offsets exceed the dynamic range of the gain-controlled amplifier. Consequently, the cancellation of the DC offsets in the gain-controlled amplifier is a very important issue.
As a circuit for canceling the DC offsets of the gain-controlled amplifier, for example, a circuit for detecting a DC offset voltage when no signals exist to perform a correction on the basis of the detection result is known (see, for example, Japanese Patent Application Publication No. Hei2000-216836). Specifically, the amplification factor of the gain-controlled amplifier is controlled for a predetermined period immediately after turning on the power source thereof to generate a no-input state to a quadrature detector, and the inputs to a demodulator during a period in which the no-input state is lasting are averaged. Thus, the adjustment quantity of the DC offset in the demodulator is determined.
By the above-mentioned prior art technique, for example, in the case of a system in which a no-signal state exists, such as the Global Systems for Mobile Communications (GSM) system for a mobile telephone, it is possible to correct the DC offset every time of no-signal periods by using the no-signal periods. However, in the case of a system in which a receiving operation is continuously performed, such as the Wide-band Code Division Multiple Access (W-CDMA) system, the DC offset cancellation using the no-signal period cannot be performed. Moreover, a change of the DC offset owing to the operation conditions, such as temperature, cannot be corrected.
The present invention was made in view of the above-mentioned problems, and it is an object of the invention to provide again-controlled amplifier capable of dealing with the system for performing the receiving operation continuously, and capable of correcting a change of a DC offset owing to an operation condition such as temperature; a receiver circuit using the gain-controlled amplifier; and a radio communication device installing the receiver circuit.